Personal alarm systems incorporate transmitters worn by users. The transmitters include a trigger mechanism that the user may activate, either intentionally, unintentionally, consciously or unconsciously, to broadcast an alarm signal that may be received by a receiver. The receiver then annunciates the alarm so that an appropriate response may be made.
One class of alarm system broadcasts alarm signals through the use of ultrasonic or infrared signals. Such systems are advantageous because ultrasonic and infrared signal receivers will detect alarm signals transmitted from only a small distance. Thus, in a building, one receiver may be placed in each room of interest, and an alarm signal may be easily localized by identifying the particular receiver detecting the alarm signal. The number of receivers required for wide area detection makes the system unacceptably costly. Ultrasonic signal receivers are unacceptably prone to false alarms while infrared signal receivers are unacceptably prone to failures in detecting valid alarm signals.
Radio frequency (RF) location systems are also known, which rely upon RF signals to localize a locatable unit through the use of such techniques as multilateration, triangulation etc. RF signals travel at the speed of light and often travel in poorly controlled directions depending on antenna patterns, reflections etc. RF signals are typically used in location systems operating over large distances so that the problems of interference, multipath and false alarms are mitigated.
When conventional RF location systems use multilateration to localize a locatable unit, they require precise synchronization between locator units so that precise timing measurements may be made for RF signals propagating between the locatable and locator units, typically requiring a complex and dedicated communication network among the locator units. Unfortunately, many applications that might benefit from the use of an RF personal alarm and location system cannot support the installation expense of such a communication network.